Book pages are bound together, usually by stitching and glue, and attached to a protective cover. Brochures as small booklets of only a few pages which are now stapled together, but, before there were staples, the booklets were bound by stitches or strings.
The French word 'brocher' (pronounced 'broh-SHAY') meant 'to stitch'. 'Brochure' was an item which was stitched together, making a small book. A small book is called a 'booklet'.